The present invention relates to a focus servo apparatus for use in an optical read-out system of information recorded on an optical video disk and, in particular, to a focus servo apparatus for a focusing lens of an information read-out system.
The optical video disk is usually in the form of a circular disk plate (6) made of a transparent material on one surface (7) of which a plurality of recesses (17) called pits are formed on and along a spiral track or on a plurality of circular tracks arranged coaxially as shown in FIG. 1a. The information is constituted by the length of the pit and the distance between the adjacent pits. In order to improve the reflectivity of the surface, a reflection layer is provided on the surface by vapor-depositing aluminum. A protective layer (18) is provided on the vapor-deposited aluminum layer, as shown in FIG. 1b which is a cross section of the disk in FIG. 1a.
A read-out of the information recorded on the disk is usually performed by directing a light to the other surface of the disk, detecting light reflected from the reflection layer on the one surface, which is modulated by pits, and demodulating the detected light.
In a read-out system of this type, a servo mechanism is necessary to control a lens system for focusing the incident light precisely on the reflection surface (7).
An example of the conventional focus servo apparatus is shown in FIG. 2, in which a light beam emitted from a light source (1) such as a helium-neon laser is passed through a collimater lens (2), a beam splitter (3) and a movable mirror (4) and condensed by a condenser lens 5 at a point near the recording surface (7) of the disk (6). The disk (6) is rotated at high speed by a motor (14). The reflected light containing the information recorded on the disk passes through these optical elements in the reverse direction, and is split by the beam splitter (3) and converted by a suitable photo-electric conversion element into an electric signal.
It is impossible, however, to fabricate the disk with complete flatness and, even if the disk is completely flat, it may be impossible to mount it on a shaft of the motor 14 without some tilting. Therefore, when the disk is rotated under such a tilted mounting condition, the disk may be vertically fluctuated.
In order to read-out the information on the fluctuating disk exactly, the condenser lens 5 must be moved vertically to follow the fluctuation to thereby condense the light at the exact point on or adjacent the recording surface 7 of the disk 6 during read-out. To this end, a cylindrical lens 8 is provided to receive the split light from the beam splitter 3. The light passed through the lens 8 is received by a light receiving element 9. The light receiving element 9 comprises four segments 9a, 9b, 9c and 9d as shown in FIG. 4b. The segments are arranged such that a straight line A connecting centers of light receiving faces of the segments 9a and 9b and a straight line B connecting centers of light receiving faces of the segments 9c and 9d are orthogonal to each other and either one of the straight lines is oriented in the same direction as a center axis of the cylindrical lens 8. Due to the fact that the focus positions of a light beam passed through the lens 8 in a plane including the lens 8 and in a plane orthogonal to the lens plane are different, contours of light bundles projected onto the light receiving faces of the segments 9a-9d of the light receiving element 9 are different, so that the positional relation between the recording surface 7 and the condenser lens 5 can be determined by using the difference in the output of the element 9.
That is, the light receiving surface of the element 9 is positioned so that when the focus point of the lens 5 is at a position in the plane of the recording surface of the disk, the contour of the reflection light after being passed through the lens 8 becomes substantially square (FIG. 4b).
With this position of the element 9, outputs Va, Vb, Vc and Vd of the respective light receiving segments are equal and, thus, the following equation is established: EQU Va+Vb=Vc+Vd
Accordingly, an output V of a differential amplifier 10 having differential inputs Va+Vb and Vc+Vd as shown in FIG. 3 becomes zero. Consequently, both the output of amplifier 11 and the output of a lens driving device 12 are zero and therefore the position of the lens is not changed.
In case the incident light is focussed behind the recording surface as shown by a dotted line in FIG. 5a, that is, the distance between the recording surface 7 and the condenser lens 5 is too small, the contour of the light in the light receiving plane of the element 9 is as shown in FIG. 5b. In this case, the sum of the output voltages Va and Vb of the segments 9a and 9b becomes larger than the sum of the output voltages Vc and Vd of the segments 9c and 9d, and, therefore, the output of the differential amplifier 10 becomes negative. On the other hand, in case the incident light is focussed in front of the recording surface 7 as shown in FIG. 6a, the cross sectional contour of the light incident on the receiving element 9 becomes as shown in FIG. 6b. Therefore, the sum of Va and Vb becomes smaller than the sum of Vc and Vd and the output V of the differential amplifier 10 becomes positive. Accordingly, assuming that Z=0 when the focussing position is in the recording surface 7 and Z&gt;0 when the focussing position is shifted towards the condenser lens, the output V of the differential amplifier 10 can be represented by a dotted curve 20 in FIG. 7. This output of the differential amplifier 10 is amplified by the amplifier 11 as an error signal which is supplied to the driving device 12 to control the position of the condenser lens 5 through a holder 13 thereof.
In the conventional automatic focus servo device as mentioned above, when the movable mirror 4 or the condenser lens 5 is precisely positioned and the light is focussed on the track of the pits 17, i.e., when the tracking is properly performed, the track of the pits 17 is projected on and along the straight line connecting the centers of the segments 9a and 9b or 9c and 9d and the pits form elongated dark marks. As long as the tracking is performed properly, the output of the light receiving element 9 is maintained at zero if the focusing of the lens is on the recording surface. However, it is very difficult to maintain the tracking properly and thus the changing rates of values Va+Vb and Vc+Vd are usually small and different, causing an erroneous focus servo operation.
Therefore, in order to maintain the proper positional relationship between the light receiving surface of the light receiving element and the condenser lens, it is necessary to exclude the effect of the improper tracking.